Some of the common causes of breaking cotton are :-
 

• A needle in the wrong way round or not aligned accurately
• A bent or blunt needle
• Sharpness in the needle hole of the needle plate

A bent or blunt needle, as well as causing a slipstitch, will also often cause the thread to break. If in doubt, fit a new needle and make sure it is correctly installed.

If needles have broken in the past, they may have left sharp grooves in the hole of the needle plate, try to rub these out with emery cloth.